The present disclosure relates to methods and apparatus for material processing using a plasma thermal source.
Glass substrates may be used in a variety of applications, including windows, high-performance display devices, and any number of other applications. The quality requirements for glass substrates have become more stringent as the demand for improved resolution, clarity, and performance increases. Glass quality may, however, be negatively impacted by various processing steps, from forming the glass melt to final packaging of the glass product.
One processing step that may result in reduced glass quality is the melting process, wherein glass batch material are mixed and heated in a melting apparatus. During this process, the glass batch material melt and react, giving off reaction gases, which produce bubbles in the molten glass. Additionally, the melting process may produce an inhomogeneous glass melt having regions of differing chemical compositions. The first melt to form is often highly reactive with the refractory materials, which may lead to excessive wear of the apparatus and/or defects in the glass melt. Denser portions of the melt may also sink to the bottom of the melting apparatus, leading to a sludge layer, which has different optical properties than the rest of the melt and is difficult to completely mix back into the overall melt. The sludge layer therefore results in inhomogeneous portions of the melt, referred to in the art and herein as chord. Finally, due to typically large processing volumes, it is possible that various glass batch material may not completely melt. Any unmelted or partially melted materials are carried through the melting process and may later become defects in the glass product.
Current melting processes for producing high quality optical glass utilize high temperatures and stirring to remove bubbles from the glass melt. However, such processes may be cost prohibitive, as they require expensive metals and specially designed high temperature refractory materials for the processing equipment. Further, these costly melting systems require a long processing time and high energy expenditure as the reaction gases have a long distance to travel to escape the glass melt and the sludge layer must be mixed from the bottom of the melter tank into the rest of the glass melt in the tank, requiring a mixing motion over a long distance through a highly viscous fluid.
Alternative methods for preventing glass bubbles and inhomogeneous portions in the glass melt include processing the melt in smaller batches. In this manner, the gas bubbles have a shorter distance to travel to escape the melt and the sludge layer can be more easily incorporated into the rest of the melt. However, as with many small scale processes, these methods have various drawbacks such as increased processing time and expense.
Accordingly, there are needs in the art for techniques to improve the melting processes of glass batch material for producing high quality optical glass.